Redundant switching arrangement

ABSTRACT

The invention relates to a redundant switching arrangement using a first and second switching network (SWF_A, SWF_B) or other switching element. The switching elements are used to carry out switching operations in the same way, so that one of the switching elements serves as an active switching element whose switching operations are utilised, while the other switching element serves as a passive switching element backing up the active switching element. To ensure that any faults occurring in the switching elements in such a redundant switching arrangement are detected as quickly as possible, their operation is monitored by comparing the data of the corresponding output channels of the first and second switching elements, and if such a comparison shows that the data contained in any of the corresponding output channels are not identical, an internal test is carried out for at least one of the switching elements to verify the data of the output channel involved. The internal compare test(s) is/are used as a basis for selecting the switching element to continue to serve as the active switching element.

This application is a continuation of international application serialnumber PCT/FI99/00175, filed Mar. 5, 1999.

FIELD OF THE INVENTION

The invention relates to providing redundancy for switching operationsregularly carried out in telecommunications systems. In particular, theinvention relates to switching elements backed up by redundant units,and specifically to redundant switching networks. In this context, aswitching element refers to any unit or system that carries out aswitching operation.

BACKGROUND OF THE INVENTION

For example at telephone exchanges, a switching network is the mostimportant single component whose failure may, in the worst case,paralyse the telephone services of a larger number of subscribers.Therefore it is vital that the operation of the switching network can beefficiently controlled and that the operating personnel are immediatelynotified of any malfunctions to ensure that such malfunctions arequickly located and repaired.

Traditionally, the operation of the switching network has been protectedby using two parallel switching networks that serve as mutual spareunits. FIG. 1 illustrates this type of switching network arrangementwith two parallel switching networks, SWF_A and SWF_B. Normally, thedata received at the switching network are connected to the input portsof both switching networks (INAi and INBi, i=1,2, . . . n), and bothswitching networks operate all the time carrying out switchingoperations in the same way. As a result, the data fed to the outputports (OUTAi and OUTBi, i=1,2, . . . n) are in normal operationidentical. However, only one of the switching networks is, at any giventime, selected as the active switching network whose output signals areforwarded.

The operation of an individual switching network (SWF_A or SWF_B) ismonitored by performing internal comparative testing controlled by theswitching network control unit (CU_A, CU_B). This comparison is carriedout (see arrows) by branching off the data of selected output channels(time slots) and that of corresponding input channels (time slots) tothe control unit which compares the two sets of data for the duration ofseveral frames. Considering the total number of time slots, the actualnumber of channels being compared simultaneously is normally very low toensure that the system does not become too complicated.

One drawback of such a redundant system is that whenever a minor faultoccurs in the equipment that induces errors in the data passing throughthe switching network, this is not noticed until the internal comparetest of the switching network happens to compare the input and outputchannels of that particular switching operation. For example, in aswitching network with a maximum capacity of 2048 PCM circuits (20482048-kbit/s PCM signals, PCM=Pulse Code Modulation), it takes tens ofseconds to identify an error in any single channel.

Another known method of providing redundancy is to use three switchingnetworks in parallel and to compare the output data of all the switchingnetworks. By applying the majority vote principle, the system identifiesthe switching network(s) that work(s) correctly. However, this is anexpensive solution because it requires three identical switchingnetworks.

SUMMARY OF THE INVENTION

The purpose of the invention is to eliminate the said drawbacks and toprovide a solution that allows the operation of a switching network orother switching element to be tested quickly and reliably without havingto resort to costly hardware.

This goal is achieved by using the solution defined in the independentpatent claims.

The idea of the invention is to provide redundancy for a switchingnetwork (or other switching element) by doubling and to test theoperation of the switching networks by comparing data in correspondingoutput channels, preferably on a continuous basis. If this firstcomparison shows that the data in certain corresponding output channelsare not identical, one of the switching networks is not operatingcorrectly. Then, an internal compare test is carried out in at least oneof the two switching networks. Because the first compare test hasalready identified the output channel where the error occurred, theinternal compare test can be carried out on that particular channel andthe corresponding input channel data. Thus, the actual internal comparetest is carried out using a known method, but now the affected channelscan be selected for comparison immediately. As a result, the outcome ofthe compare test is obtained immediately and the passive switchingnetwork can be quickly activated, if the compare test(s) show(s) thatthe currently active switching network is faulty.

Another advantage offered by the solution in accordance with theinvention is that the inter-network compare test to be added to a knownswitching network arrangement that uses an internal compare test can becarried out very simply. This is due to the fact that the comparisonbetween the switching networks can be performed without having to savethe comparison data because the corresponding output channels areconcurrently present at corresponding outputs of the switching networks.

In one preferred embodiment of the invention, the validity of theconnection information of both switching networks is checked after theinter-network compare test has revealed a discrepancy relative to theoutput data from the switching networks. For example, this can beeffected by having the control unit read the connection information usedby the switching network and compare this information against its ownconnection information. If the connection information of both switchingnetworks is correct, this is followed by an internal compare test in atleast one of the switching networks. If not, an attempt may be made toreplace the incorrect connection information by correct data, andfailing that, an alarm can be given.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention and its preferred embodiments areexplained in greater detail with reference to FIGS. 2 through 8 based onthe examples given in the drawings, where

FIG. 1 illustrates a known redundant switching network;

FIG. 2a illustrates a redundant switching network in accordance with theinvention;

FIG. 2b illustrates comparison of corresponding channels;

FIG. 3 is a flowchart illustrating the method in accordance with theinvention;

FIG. 4 illustrates the functional blocks of a known telephone exchange;

FIG. 5 illustrates an implementation of the switching network at thetelephone exchange shown in FIG. 4;

FIG. 6 shows the telephone exchange of FIG. 4 with a backed-up switchingnetwork;

FIG. 7 illustrates the performance of the compare test between switchingnetworks in the switching network shown in FIG. 5; and

FIG. 8 illustrates the performance of the internal compare test in theswitching network shown in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

In the following, a solution according to the invention is firstdescribed on a general level with reference to FIGS. 2a, 2 b and 3. FIG.2a illustrates the switching arrangement shown in FIG. 1 with redundantswitching networks, while FIG. 3 is a flowchart illustrating the stepsof the method according to the invention. As suggested by the invention,the operation of the switching networks is monitored by comparing thedata in two corresponding output channels (comparators C1 . . . Cn, FIG.2a, and step 31, FIG. 3). FIG. 2b illustrates the comparison of twocorresponding output channels assuming that transmission is carried outin successive time slots the number of which per each transmission frameFR is pre-defined. The frames appear in the corresponding output portsin the same phase, and so the corresponding time slots (channels) occursimultaneously and comparison can be carried out without having to savethe data in between.

Preferably, comparison between the switching networks is carried out ona continuous basis and if no discrepancy is detected, a new compare testbetween the switching networks is performed. If a discrepancy isdetected between one or more output channel pairs, an internal comparetest (FIG. 3, step 33) is carried out in one, but preferably both,switching networks in order to identify the incorrectly operatingswitching network.

In an internal compare test, the data from the faulty output channel andthe data from the input channel corresponding (according to theswitching operation used) to that output channel are branched off to thecontrol unit from the input and output ports involved. The switchingnetwork where the input and output data differ from each other isfaulty.

Once the internal compare test has identified the faulty switchingnetwork, the active switching network is deactivated (step 35) and thepassive switching network activated, if the fault exists in the activeswitching network. If, on the other hand, it is determined that thefault lies in the passive switching network, the active switchingnetwork is allowed to remain active. Interchanging the switchingnetworks can be effected by having the control unit send a signal to theswitching networks and connected units to indicate which switchingnetwork is currently active. Thus, the units involved will know theswitching network whose signals should be allowed to pass through.

After these steps have been taken, the fault will exist in the passiveswitching network, making it possible to identify the faulty plug-inunits (one or several) (step 36). Following repairs to the defectivecomponents (step 37), compare testing between the switching networks canbe resumed.

When a discrepancy is detected in one or more output channel pairs, itis possible first to check the switchings before any internal comparetest(s) is/are initiated. If the connection information are found to becorrect, an internal compare test will be started in at least one of theswitching networks. If the connection information are found to beincorrect, the control unit will make an attempt to replace theincorrect connection information with correct data. Failing that, theoperating personnel will be alerted. If the connection information canbe successfully fixed, compare testing between the switching networks isresumed.

A more detailed description of one embodiment of the invention isprovided below using the switching arrangements common in theconventional TDM network as examples.

In conventional TDM (Time Division Multiplexing) networks, data aretransmitted as a bit or symbol stream in time slots, each containing acertain number of bits, typically eight. In conventional PCM systems,these bits in any single time slot are all reserved to one and the samechannel. For example, in the European 2048 kbit/s basic multiplexingsystem (where the frame length is 32 time slots, i.e. 256 bits), it ispossible to transmit a total of 30 voice channels, each with a capacityof 64 kbit/s. (In the corresponding U.S. system, the number of channelsis 24 and the transmission rate 1544 kbit/s.) A description of asolution based on the invention is provided below, adapted to atelephone exchange in a TDM network.

FIG. 4 illustrates the functional blocks of a telephone exchange systemas used, for example, in the applicant's DX 210 telephone exchange. Thecore of the exchange consists of the switching network SWF whose task isto interconnect input and output channels. Switching takes place underthe control of the call control unit CAC. The call control unit CAC isin charge of all decision making relating to call control. The larger DX220 exchange, also manufactured by the applicant, makes use ofdecentralised call control by distributing the functions required forcall control between several computer units. Data between these unitsare transmitted via the message bus MB. Operation and maintenance of theexchange is effected by means of the operation and maintenance unit OMU.Peripheral equipment such as display terminals and printers areconnected to the operation and maintenance unit.

Subscribers are connected to the exchange by means of subscriber modulesSUB. The interface can be a standard analog interface or a digital ISDNinterface. The subscriber modules carry out the A/D and D/A conversionsrequired for analog interfaces and handle signalling operations betweenthe subscriber terminal (telephone) and the exchange.

The exchange is connected to other exchanges or remote subscriber stagesby means of trunk circuit interfaces ET. The external interface of theexchange conforms to the CCITT (currently ITU-T) G.700 seriesrecommendations.

In a system as described above, the switching network SWF can, forexample, be designed as shown in FIG. 5. The design is identical to thatused in the applicant's DX 210 and DX 220 exchanges. The structuralcomponents of the switching network are the converter units SPSi (i=1 .. . 32) and switching blocks SWE_(ij) (i=1 . . . 8, j=1 . . . 8). Inpractice, the control unit of the switching network consists of fourplug-in units, but, for the sake of simplicity, the figure only showsone shared control unit CU (which is part of the call control unit CACshown in FIG. 4).

Data are transmitted between the subscriber modules SUB and theconverter units via the 4 Mbit/s serial buses SB, numbering 32 perconverter unit. Each 4 Mbit/s serial bus contains the contents of two 2Mbit/s PCM circuits multiplexed on a time division basis (64 kbit/schannels). The data used for testing in accordance with the inventionare branched off at this 4 Mbit/s interface, as will be explained below.As a result, the capacity of the switching network used as the exampleis 2×32×32=2048 PCM circuits (2048 2048 kbit/s PCM signals).

The 4 Mbit/s serial buses from the subscriber modules are converted byserial-to-parallel converters into a single parallel bus with a capacityof 16.384 Mbytes/s. The parallel bus IBi (i=1 . . . 32) from eachconverter unit connects to eight switching blocks shown in the figure onthe same horizontal line. The parallel buses from converter units SPS1 .. . SPS4 connect to switching blocks SWE_(i1) (i=1 . . . 8), those fromconverter units SPS5 . . . SPS8 to the switching blocks SWE_(i2) (i=1 .. . 8), and so on, and the parallel buses from converter units SPS29 . .. SPS32 to switching blocks SWE_(i8) (i=1 . . . 8). Consequently, thereare a total of four parallel buses from four separate converter units toeach switching block (one from each converter unit).

Conversely, there are a total of four (16.384 Mbit/s) parallel busesfrom each switching block to four separate converter units (one to eachunit). The switching blocks shown in the figure on the same verticalline drive the same output bus that connects to one converter unit. Inother words, switching blocks SWE_(1j) (j=1 . . . 8) drive bus OB1 thatconnects to converter unit SPS1, bus OB2 that connects to converter unitSPS2, bus OB8 that connects to converter unit SPS3, and bus OB4 thatconnects to converter unit SPS4, while switching blocks SWE_(2j) (j=1 .. . 8) drive bus OB5 that connects to converter unit SPS5, bus OB6 thatconnects to converter unit SPS6, bus OB7 that connects to converter unitSPS7, and bus OB8 that connects to converter unit SPS8, and so on, andswitching blocks SWE_(8j) (j=1 . . . 8) drive bus OB29 that connects toconverter unit SPS29, bus OB30 that connects to converter unit SPS30,bus OB31 that connects to converter unit SPS31, and bus OB32 thatconnects to converter unit SPS32.

At each individual converter unit, the 4 Mbit/s serial buses (numbering32) are formed from the parallel bus coming from the switching blocks byperforming a parallel-to-serial conversion on the incoming data.

Thus, each converter unit transmits to eight different switching blocks(shown on the same horizontal line in the figure) and receives fromeight switching blocks (shown on the same vertical line in the figure),making it possible to switch any incoming time slot to any outgoing timeslot.

Control unit CU connects directly to each converter unit and eachswitching block via control bus CB.

The switching blocks SWE_(ij) receive and transmit data via the saidparallel buses. In the switching block, the incoming parallel data arewritten cyclically, byte by byte, to the switching memory. In theoutgoing direction of transmission, data transmission is controlled bythe control memory in the switching block, to which the control unit haswritten, via the control bus, the road address for the switchings to bemade. The control memory is read cyclically in time with the outgoingtime slots. The read address in the control memory indicates the addressin the switching memory from which the data for the time slot involvedshould be read. Thus, the actual cross-connection is effected using aknown method.

When the method in accordance with the invention is used, the switchingnetwork, as described above, is backed-up by providing redundancy asshown in FIG. 6, where there are two parallel switching networks (SWF_Aand SWF_B) as shown in FIG. 5, one of which is selected for activeoperation. The units (SUB, ET) connecting to the switching networkselect the data transmitted by the active network. In this example, bothswitching networks have their own control units.

FIG. 7 illustrates one way of applying inter-network comparison to thetype of switching network described in the foregoing. As indicatedabove, this comparison can, for example, be carried out in the converterunits. All the functions in the various converter units are identical,so that the following explanation applies to both switching networks. Inthe figure, the elements belonging to the first switching network(SWF_A) are denoted by the letter A and the elements belonging to thesecond switching network (SWF_B) by the letter B.

The PCM signals from an individual converter unit towards the subscribermodule, numbering 32 in this example, are denoted by T0 . . . T31. Forcomparison, these signals are branched off to a separate multiplexer M1to whose output one incoming signal at a time is connected for theduration of one frame (125 μs). The multiplexer output is connected bothto the first input of the own comparison unit COMP and to the secondinput of the comparison unit in a converter unit belonging to the otherswitching network. In this example, the converter units SPS_A and SPS_Bare interconnected by two 4 Mbit/s interfaces. Multiplexer M1 iscontrolled by a branching logic unit BLU, which is used to select thesignal to be compared at any given time. The branching logic units aresynchronised to ensure that they always select the same signal.

Comparison unit COMP compares corresponding channels during one frame,after which it analyses the corresponding channels of the following (4Mbit/s) signal. Thus, the comparison unit carries out comparison for 64channel pairs during a single frame. As a result, all the channels canbe checked during 32 frames. This also means that an error in any signalis detected within a period of time equivalent to the duration of 32frames, if not earlier (which is a short time compared with theconventional solution). The serial data coming in from multiplexer M1are stored, after which comparison is carried out byte by byte (channelby channel) using a known method. The comparison unit indicates whetherthe bytes in the corresponding channels differ from one another. Thisitem of data is written to the buffer memory BM together with the dataindicating the channel involved. The latter data are obtained from thebranching logic unit. Thus, the buffer memory receives information,channel by channel, as to whether the contents of each channel areidentical in both switching networks.

Control unit CU reads the contents of the buffer memory via control busCB, and if it detects an error (discrepancy) in any outgoing channelpair, it starts the internal compare test for the switching networks.Because the control unit is aware of the switchings to be made, it iscapable of targeting the compare test to the output channel suspected offailure and the corresponding input channel. The control unit reads thecontents of the buffer memories of all the converter units during oneframe.

In the examples detailed above, 1 signal out of 32 is selected at anyone time for comparison. This helps to achieve a compromise between thespeed of detection of a fault and the complexity of the equipment. Iffaults are to be detected more quickly, several signals must be comparedsimultaneously. This also calls for increased data transmission capacitybetween the converter unit and the control unit.

Because both switching networks basically include identical hardware,the compare test between the switching networks can be carried out inboth switching networks. However, since the output channels of only theactive switching networks are used, it is natural that use is only madeof the compare test result of the active switching network. Thus, thecontrol unit of the active switching network gives the control unit ofthe passive switching network the command to start the internal comparetest within that switching network. However, a compare test between theswitching networks can only be performed in one of the switchingnetworks (the same applies to the internal compare test within theswitching network).

The principle of the internal compare test is illustrated in FIG. 8 forone converter unit SPSi. The test works in the same way for otherconverter units also.

Comparison between the input and output channels is carried out byswitching network control unit CU, not by the converter unit, as in thecompare test between the switching networks. To start comparison, thecontrol unit gives a branching command via control bus CB to theconverter unit SPSi corresponding to the input and output channel (i=1 .. . 32 and may have 1 or 2 values). In response to the branchingcommand, the converter unit branching logic BLU branches off the timeslots involved to the control unit via the control bus. In the controlunit, comparison logic CL compares the contents of the input and outputchannels corresponding to the switching involved. The comparison logicreports the results of the test to the control unit microcomputer MCwhich makes the decision on the further action required.

Testing of the faulty passive switching network can, for example, becarried out using the through-connection test. This test differs fromthe internal compare test of the switching networks in that theconverter unit transmits a test byte assigned by the control unit to thetime slots coming into the switching network. The control unit startsthe through-connection test by writing the test byte to the converterunit test register (not shown) via control bus CB. The outgoing timeslots of the connection to be tested are branched off from the converterunit via the control bus to the control unit whose comparison logiccompares the contents of the outgoing time slot to the test bytetransmitted to the converter unit corresponding to the incoming timeslot. In other words, branching and comparison of time slots is carriedout using the same principle as in internal compare testing, except thatincoming time slots are not branched off to the control unit because thecontents of the incoming time slots (test bytes) are already known tothe control unit.

FIGS. 7 and 8 show, on a general level, only those components that areessential to the performance of the tests. For example, in addition tobranching logic, FIG. 8 only shows, for the converter unit, theinterfaces that carry out the parallel-to-serial conversion (PISO) onthe side of the switching blocks, and the serial-to-parallel conversion(SIPO) in the opposite direction of transmission. Moreover, the samebranching logic (BLU) is shown performing branching for both tests.

Both in the compare test between switching networks and in the switchingnetwork internal compare test, it is preferable to carry out thecomparison by comparing the bytes or bit sequences involved directly.However, it is also possible to perform, this comparison indirectly byusing some function to compute a provisional result, such as a checksum, from the bytes or bit sequences to be compared and then comparingthese provisional results. The same applies to the through-connectiontest.

Although the invention has in the foregoing been explained withreference to the examples shown in the attached drawings, it is clearthat the invention is not limited to these embodiments but can be variedwithin the scope of the basic idea of the invention as presented in theenclosed patent claims. For example, even though the principle based onthe invention has been described above in connection with a switchingnetwork used in a TDM network, the same principle can be used to provideredundancy for a switching network in a packet-switched network, such asATM network. In such a case, the packets or cells of one virtualconnection are compared at corresponding output ports, and if anydiscrepancy is detected, the packets (or cells) are then compared at theoutput and input ports of both switching networks (the packet present atthe input port is compared with the same packet at the output port).Also, the compare test between switching networks or the internalcompare test need not necessarily be carried out in both switchingnetworks, but both tests can be carried out in only one of the twoswitching networks. If the (internal) test performed in the switchingnetwork (or in its control unit) gives the correct result, it is clearthat the fault lies in the other switching network (and vice versa).Comparisons can also be carried out outside the actual switchingnetwork, as explained above for the internal compare test. Nor is thesolution according to the invention tied to providing the entireswitching network, but the same principle can be applied to any elementperforming switching operations, such as a switching element in theswitching network. Therefore, the term ‘switching element’ used in theenclosed patent claims must be understood as covering a range ofoptions. Switching networks (or elements) may also share a singlecontrol unit backed-up by a redundant unit.

What is claimed is:
 1. A method for implementing a redundant switchingarrangement, the method including the steps of using a first and asecond switching element (SWF_A, SWF_B), both of which include at leastone input port and at least one output port, and performing switchingoperations by means of the two switching elements in an identicalmanner, so that one of the switching elements operates as an activeswitching element whose switching operations are utilised and the otherswitching element operates as a passive switching element serving as aspare unit for the active switching element, characterized in that theoperation of the switching elements is monitored by comparing the dataof the corresponding output channels of the first and second switchingelements, and if the comparison shows that the data contained in any twocorresponding output channels are not identical, performing an internaltest on at least one of the switching elements to verify the data in theoutput channels involved, and selecting the switching element serving asthe active switching element on the basis of such internal test(s).
 2. Amethod according to claim 1, characterized by comparing the data of thecorresponding output channels of the first and second switching elementessentially continuously.
 3. A method according to claim 1,characterized in that, in the internal test, the data of the outputchannel involved are directly compared with the data of thecorresponding input channel.
 4. A method according to claim 1,characterized in that when comparison reveals discrepancies in thecontents of certain corresponding output channels, the correctness ofthe connection information is first verified in both switching elementsbefore any internal test is carried out, and said at least one internaltest is performed only if the connection information prove to becorrect.
 5. A method according to claim 1, characterized in that thedata of the corresponding output channels of the first and secondswitching element are compared at least in one of the switchingelements.
 6. A method according to claim 1, characterized in that thedata of the corresponding output channels of the first and secondswitching element are compared in both switching elements, but only theresult of comparison provided by the active switching element is used asa basis for decision-making.
 7. A method according to claims 1,characterized in that, for the comparison of the data of thecorresponding output channels, some of the output channels in bothswitching elements are branched off to be compared, and that thechannels to be branched off are changed cyclically.
 8. A redundantswitching arrangement for a telecommunications network, the switchingarrangement comprising a first switching element (SWF_A) including atleast one input port and at least one output port, and a secondswitching element (SWF_B) including at least one input port and at leastone output port and configured to perform switching in the same manneras the first switching element, and first comparison means (CL, MC) forperforming an internal compare test for at least one of the switchingelements to verify the data of the selected output channel,characterized in that the arrangement further comprises secondcomparison means (COMP_A, COMP_B) for comparing the data of thecorresponding output channels of the first and second switching elementand that the first comparison means are responsive to the secondcomparison means for carrying out an internal compare test in a waydependent on the result of the comparison performed by the secondcomparison means.
 9. A switching arrangement according to claim 8,characterized in that the second comparison means are in both switchingelements.
 10. A switching arrangement according to claim 8,characterized in that it includes multiplexing means (M1) to whoseinputs output channels are connected and whose output is connected tothe second comparison means for connecting some of the output channelsat any one time to the second comparison means, and selection means(BLU) controlling the multiplexing means for selecting the outputchannels to be connected to the output of the multiplexing means at anygiven time.